1. Technical Field
The present invention pertains to satellite communication networks. In particular, the present invention pertains to low orbit satellite communication networks that utilize apriori knowledge of network topology (e.g., as a function of time or satellite geographical location) to enhance routing efficiency and significantly increase overall network performance.
2. Discussion of Related Art
Dynamic routing in a single Autonomous System of an Internet Protocol (IP) based communication network is typically accomplished by employing an Interior Gateway Protocol (IGP), such as the Open Shortest Path First (OSPF) routing protocol. The OSPF protocol is basically an internetworking or gateway protocol facilitating communications with external networks. For examples of implementation of the OSPF Protocol, reference is made to RFC 1583, Moy, “OSPF Version 2,” March 1994, the disclosure of which is incorporated herein by reference in its entirety.
Routing is accomplished in the OSPF protocol by each network node having a routing database containing information related to network topology (e.g., links between network nodes). The routing database is utilized by each node to determine a path for transmitting a message to a destination site. The routing or path information is typically stored in a routing table. The routing databases are updated by exchanging link-state advertisement (LSA) packets between neighboring nodes. These packets generally include information related to current links of network nodes and are typically transferred periodically and/or in the event of a modification to the network topology. The OSPF protocol designates a particular router to flood LSA packets to neighbors in broadcast type networks, while LSA packets are transmitted via point-to-point and/or broadcast packets within non-broadcast type networks. Thus, the OSPF protocol is capable of ascertaining the topology of an Internet Protocol communication network and determining routes to be used by the Internet Protocol.
The OSPF protocol may further detect disabled communication links via periodic transmission and reception of neighbor discovery and maintenance or “Hello” type packets between network nodes. These packets are periodically transmitted by each node to discover neighboring nodes and to ensure communications between that node and the neighboring nodes. In addition, the OSPF protocol may determine alternative routes for current routes that utilize disabled links and are no longer viable. However, the OSPF routing protocol suffers from several disadvantages. In particular, the protocol takes a finite time interval to detect and repair a broken or disabled route. Although this is unavoidable in cases where the route becomes disabled due to an unforeseen event, protocol efficiency is less than optimal when apriori knowledge is available concerning network topology changes that affect viability of routes. In this case, protocol efficiency may be enhanced by determining alternative routes prior to a network topology change.
In an attempt to accommodate network topology changes, the related art provides various systems that determine alternative routes based on modifications to the network topology. For example, U.S. Pat. No. 5,365,520 (Wang et al) discloses dynamic signal routing. Data packets are delivered through a constellation of nodes or satellites to a termination unit. The node where a packet leaves the constellation is a terminal node. Each packet includes a routing code. When a node receives a packet, the node examines the routing code to determine if that node might be a terminal node for that packet. A table look up operation is performed using the routing code as an index to a routing table. The table identifies a link to use in routing the packet to a neighbor node. A number of different tables are used for each orbit, where the tables may be generated to track the movement of nodes over space regions. The packet is also examined to verify compatibility between packet type and a selected link. When a node concludes that the node might be a terminal node, the node evaluates a channel identifier to determine if the node is currently serving the party to whom the packet is directed.
U.S. Pat. No. 5,999,797 (Zancho et al) discloses a method and apparatus for providing private global networks in a satellite communication system. The private networks between communication terminals are established within a satellite communication system. Each private network provides users a network of dedicated communication paths that have durations exceeding the duration of the particular call. A dedicated communication path is established by determining hand-off schedules for satellite-to-terminal links for both a source and destination terminal, and by determining satellite crosslink schedules necessary to maintain the dedicated path for a duration exceeding the particular call.
U.S. Pat. No. 6,157,624 (Zancho) discloses a method and apparatus for linking terminals using private secondary service paths in a satellite communication system. The secondary paths between compatible or non-compatible communication terminals are established using terrestrial stations within a satellite communication system. Each secondary path provides users the ability to establish communication paths between compatible or non-compatible terminals. A secondary path is established using terrestrial stations that establish and maintain terrestrial-based links and satellite communication links. Terrestrial stations also perform frequency translating and data reformatting to allow non-compatible terminals to communicate with each other. Since the topology of a satellite communication system is constantly changing, maintenance of a secondary path involves satellite-to-terrestrial station hand-offs and establishment of different inter-satellite crosslinks. A prediction of system topology during the duration of the secondary path, or a portion thereof, is used to determine the satellite crosslinks.
The related art systems suffer from several disadvantages. In particular, the Wang et al system generates a stream of look up tables for each orbit to determine routing with respect to topology changes, thereby requiring significant processing and increasing system complexity. The Zancho et al and Zancho systems determine crosslink schedules that are employed by satellites to control crosslink establishment and relinquishment. Thus, these systems are required to distribute the schedules among the satellites, thereby providing additional tasks and overhead for communications. Further, since these systems control the crosslinks in accordance with the distributed schedules, the link control may interfere with and degrade performance of communication protocols employed by the systems.
The present invention basically overcomes the aforementioned problems by allowing the OSPF or other routing protocol to recompute routes prior to a known change in network topology. This is achieved by preventing the protocol neighbor discovery and maintenance or “Hello” type packets from being sent and received on a link that is about to become disabled due to a change in network topology. The cessation of these packets causes a protocol “dead” timer to expire for the link, thereby causing the protocol to consider the link disabled and recalculate routes in accordance with that consideration. Thus, new routes are determined prior to any previous routes becoming invalid due to the disabled link.